In Java the coordinate system is different from the "real" math coordinate system.
Here how looks like the Java coordinate system:
As you can see, the Y increases in the direction from top to bottom.
If you want to draw a line like 2*y - 3*x = 5 then, you can't work with the Java coordinates.
The fix is easy, we change the sign of the Java coordinates, for the Y value. So, of point B was at (100,150) in Java, then, the real coordinate will be (100,-150).
It's easy, no?
